1. Field of the Invention
The present invention relates to information data recording and/or reproducing methods for recording information data on a recording medium and reproducing such information data.
2. Description of Related Art
A partial response scheme is widely known as a scheme for recording and reproducing information data on and from a recording medium such as an optical disc, a magnetic disc, or the like.
FIG. 1 illustrates a recording/reproducing system which employs a PR(1,1) scheme, one of partial response schemes.
First in a recording system, a precoder 10 performs a predetermined logical operation on information data a(k) supplied from an information source, not shown, to convert the information data a(k) into recording data b(k) for recording which is supplied to a recording head 20.
The precoder 10, which consists of, for example, a subtractor 11, a mod2 processor 12, and a delay element 13, converts the information data a(k) into the recording data b(k) by the following logical operation: EQU b(k)={a(k)-b(k-1)}mod2 (1)
where
a(k): information data at time k; PA1 b(k): recording data at time k; and PA1 b(k-1) : recording data at the previous time (k-1). PA1 "mod" in the above equation is an operator representing a residual operation.
For example, EQU A mod B (2)
represents a residue produced by dividing "A" by "B," and the residue is output as its operation result. In this operation, it is assumed that the operation result has the same polarity as that of "B." In other words, the recording data b(k) is a residue produced by dividing {a(k)-b(k-1)} by two.
The recording head 20 irradiates recording laser light corresponding to the recording data b(k) onto a recording surface of a recording disc 30 which is driven to rotate at a predetermined velocity.
Assume herein that a so-called pit position recording is performed, where data is recorded as the presence or absence of a pit at each recording point.
For example, the recording head 20 irradiates recording laser light to the position of a recording point when the recording data b(k) is at logical level "1," and does not irradiate the recording laser light at the position of this recording point when the recording data b(k) is at logical level "0." In this case, pits Pt are formed only at the positions of recording points which were irradiated with the recording laser light, as illustrated in FIG. 2.
A reproducing head 40 in a reproducing system irradiates reproducing laser light onto a recording surface while tracing a recording track, and photo-electrically converts the reflected light from the recording surface to produce a read signal corresponding to a sequence of pits which are formed on the recording track.
It should be noted that in the PR(1,1) scheme, two adjacent recording points simultaneously exist on the same track within a range of a reading beam spot SP formed on a recording surface by the irradiation of reproducing laser light, as illustrated in FIG. 2. Thus, a read signal is produced at a level corresponding to the total sum of reflected light from each of these two recording points, i.e., the sum of recording data at each of the two recording points.
Here, when two binary recording data "0" and "1" are simultaneously read, a ternary read signal "0," "1," "2" is produced.
A level determining circuit 50 determines which of these three values a reading signal corresponds to, and supplies determination data c(k) in accordance with the determination result to a mod2 processor 60.
For example, when the levels of a read signal corresponding to the three values "0," "1," "2" are distributed about 0V (Volt), 1V, 2V, respectively, the level determining circuit 50 compares the read signal with two threshold values at 0.5V and 1.5V, samples the comparison result, and determines the levels of the ternary data.
The resulting determination data c(k) is expressed by: EQU c(k)=b(k)+b(k-1) (3)
The mod2 processor 60 produces reproduced information data d(k) by performing the following residual operation on the determination data c(k): EQU d(k)=c(k)mod2 (4)
As described above, this recording/reproducing system enables the reproduction of information data corresponding to each recording point even though the interval between respective adjacent recording points on the same track is narrower than the diameter of the reading beam spot SP. In other words, it is possible to increase the linear density by narrowing the interval between recording points.
This recording/reproducing system, however, cannot increase the recording density in the radial direction of a disc, i.e., cannot reduce the track pitch, so that the entire recording capacity of a disc cannot be increased so much.
In addition, since the foregoing PR(1,1) scheme produces a read signal at ternary levels, the level determining circuit 50 cannot correctly determine the level of the read signal if the read signal has a low S/N ratio.